As illustrated in FIG. 1, the major components of a thermal dye transfer printing system are:
1. The print head 10, which contains an array of discrete resistors to supply heat or electrodes to provide current with the heat generation via Joule heating.
2. The donor sheet 12 which consists of a thin base film carrying a dye material on one side and a slip layer on the side sliding against the print head. For Joule heating in the belt, a current return layer is required. The base has to be electrically conductive. Sheet 12 is fed between donor supply 11 and donor take-up 13.
3. A receiver material 14 (such as paper or transparency) in intimate contact with the dye side of the donor sheet.
4. A platen roller 16 required to form an intimate contact nip between the print head, the dye donor and image receiver, to enable transfer of the dye from the donor to the receiver, when the pulsed heat is generated either in the ribbon 12 or the print head 10.
FIG. 2 shows resistive ribbon printing where electrodes 18 inject current into the donor ribbon 20 where it heats the ink 22 and transfers it to the receiver 24.
A significant problem in this technology is that the dye donor members used to make the thermal prints are generally intended for single (one time) use. Thus, although the member has at least three times the area of the final print and contains enough dye to make a solid black image, only a small fraction of this dye is ever used.
After printing an image, the dye donor member cannot be easily reused, although this has been the subject of several patents. The primary reason that inhibits reuse of the dye donor members is that the dye transfer process is very sensitive to the concentration of dye in the donor layer. During the first printing operation, dye is selectively removed from the layer thus altering its concentration. In subsequent printings, regions of the donor member which had been previously imaged have a lower transfer efficiency than regions which were not imaged. This results in a ghost image appearing in subsequent prints.
The cost associated with having a single use donor ribbon is large because of the large area of ribbon required, as well as the large excess of dye remaining coated on the donor member. While this technology is able to produce high quality continuous tone color prints, it is desired to provide an approach which has all of the good attributes of thermal dye transfer imaging but without the limitations associated with single use donor members.
Some work has been done by others to accomplish similar goals. For example, U.S. Pat. No. 5,286,521 discusses a reusable wax transfer ink donor ribbon. This process is intended to provide a dye donor ribbon that may be used to print more than one page before the ribbon is completely consumed. U.S. Pat. No. 4,661,393 describes a reusable ink ribbon, again for wax transfer printing. The ink ribbon contains fine inorganic particles and low melting waxy materials to assist in the repeated use of this ribbon. U.S. Pat. No. 5,137,382 discloses a printer device capable of re-inking a thermal transfer ribbon. However, again the technology is wax transfer rather than dye transfer. In the device, solid wax is melted and transferred using a roller onto the reusable transfer ribbon.
U.S. Pat. No. 5,334,574 describes a reusable dye donor ribbon for thermal dye transfer printing. This reusable ribbon has multiple layers containing dye and binder which limit the diffusion of dye out of the donor sheet. This enables the ribbon to be used to make multiple prints. This enables the ribbon to be used to make multiple prints. The binder provides the medium through which the dye diffuses. Since the mass of dye is transferred by diffusion a continues tone can be achieved by heating the dye/binder to several levels of temperature thus providing a plurality of density levels in the print.
The cross-referenced application discloses a printing engine which includes a reusable thermal dye donor element having a base layer, and a donor layer on the base layer which contains wells which preferentially adsorb and desorb dye. The advantages of the invention described are a reusable dye donor element which reduces cost and complexity in addition to the minimization of environmental issues by a significant reduction in waste product. The reusable belt described contains the wells entirely within one layer such that the thickness of the pore layer is that necessary to act as a well. The description of the reusable belt indicates that a binder for the dye is not necessary. It would be anticipated by one familiar with the state of the art that an oleophilic dye contained in the well, when fused by heat, would behave in a manner similar to that of a mass transfer system. That is, the transfer of the dye mass would be binary since the dye is either in a fused state or unfused state. In this case, it would require a half-tone printing method to produce prints which have a plurality of density levels.